Timing of pruning can be important.
For general pruning, as long as we know where the targets are, can be done at
anytime. The old question used to be, when is the best time to severely
wound a tree. The practice of wounding trunks with flush cuts was common
practice. Now we know where the targets are we can prune at anytime.
However like a air plane there are two critical times. On take off and
landing are two times when small mistakes can create major problems. Just
as trees, when the leaves are forming (flushing out) and when the leaves are
falling. They are two times when small mistakes can mean big problems.
The cambium zone acts in ways we do not completely understand at those two time.

Timing, though, is important in pruning of
fruit trees and pollarding. Pollarding would require that the new sprouts
have the time to trap energy and put energy back in the system storage before
you remove the sprouts and leaves. Fruit tree pruning, let's say, in
Pennsylvania USA, would be best done in January and February. There is
always exceptions and more to the story.

Phenology is the study of periodic natural phenomena, or
the timing of natural processes. Trees have 5 major phenological periods:
(1) onset of growth; (2) formation of leaves or needles; (3) formation of wood
and inner bark; (4) storage of energy; and (5) dormancy.
Organisms associated with trees also have phenological
periods: (1) onset of growth; (2) maturation; (3) reproduction; (4) migration to
new habitats or expansion of old habitats; (5) resting stage. Some of the
tree associates are beneficial to the tree and some are potentially destructive.
Some of the microassociates may complete the 5 stages in minutes when conditions
are proper for them. Most destructive associates are most active during
tree periods 1, and between 4 and 5.
The periods are like themes. There are many variations
on the themes. The periods are affected greatly by environmental factors.
Trees in cold climates will have different phenological patterns from trees in
hot climates.
Let us look more closely at the 5 tree periods.
Period 1 is the time when nonwoody roots, mycorrhizae mostly,
are absorbing water and elements (not food) from the soil. Nitrogen and
all other elements that enter the tree must be in a soluble inorganic form.
Organic molecules contain carbon, and usually the carbon is bonded to hydrogen
and oxygen. Inorganic molecules do not contain carbon. When organic
substances are added to the soil, the microorganisms alter the substances and
inorganic soluble substances are the results. When nitrogen enters the
tree in an inorganic form, most of the nitrogen combines with carbohydrates to
form amino acids. Amino acids are building blocks for protoplasm.
The end result is more tissue.
Period 2 is the time leaves and needles (henceforth called
leaves) are forming. As leaves are forming from buds that formed the
previous year, new buds are forming in the axils of the leaves or at tips in
some conifers.
Period 3 is the time wood and inner bark are forming.
Some trees form flowers and fruit in periods 2 and 3. Fruit on many trees
begin to mature in period 4.
Period 4 is the time energy reserves as starch and fats are
being deposited in living cells in sapwood. Some reproductive parts begin
to form in period 4. The male cones and strobili form and they will be
ready to produce pollen at the end of period 1 the next year. The current
increment of wood begins to store starch and fats during the latter part of
period 4.
Period 5 is the time leaves are shedding. Nonwoody
roots begin to shed near the end of period 4 and they continue to shed during
the beginning of period 5. As the old nonwoody roots are shedding, new
nonwoody roots are forming. The fungi that are the associates of the
nonwoody roots usually produce sporophores or mushrooms during the beginning of
period 5. In cold climates, the growth of nonwoody roots slows during the
middle of period 5. The nonwoody roots begin to grow again at end of
period 5.

Note from webmaster: At about the time of Dr. Shigo's death, he
was convinced there is much going on beneath frozen soil, and in soil under ice
and water in period 5. Many of us witnessed such root and mycorrhizae
activity at the winter workshops in Portsmouth, NH. [JAK].
Click here for a very good article on just
that.